Magnetic bubble memories are well known in the art. Two well-known techniques for moving magnetic bubbles in such memories are commonly referred to as "conductor-access" and "field-access" drive arrangements. The conductor-access type bubble memory is described in A. H. Bobeck et al., U.S. Pat. No. 3,460,116, issued Aug. 5, 1969. The field-access type memory is described in A. H. Bobeck, U.S. Pat. No. 3,534,347, issued Oct. 13, 1970.
The conductor-access type memory comprises a layer of material in which magnetic bubbles can be moved, usually a garnet material grown epitaxially on a nonmagnetic garnet substrate. Several patterns of discrete electrical conductors are formed adjacent the epitaxial layer with appropriate insulating layers therebetween. Typically, three undulating electrical conductors are arranged in positions offset from one another along a path of bubble propagation. The conductors are pulsed in a three-phase manner, as is taught in the above-mentioned patent, in such conductor-access arrangements. The conductor-access type memories can be seen to require intricate electrical conductors which cross one another and are subject to shorts, as well as defects, which render manufacturing yields of such memories low. They also typically require relatively high drive power.
R. F. Fischer, U.S. Pat. No. 3,564,518, issued Feb. 16, 1971, discloses a two-phase conductor-access bubble memory employing two levels of patterned electrically-conducting material and offset permalloy elements for determining directionality of bubble movement. The permalloy elements are disposed to provide low energy or rest positions for bubbles in positions offset from those to which bubbles are moved by a pulse applied to one of the conducting levels. Thus, the permalloy is operative as a "third-phase" conductor would be if it were present.
Similarly, U.S. Pat. No. 3,693,177, issued Sept. 19, 1972, and U.S. Pat. No. 3,678,479, issued July 18, 1972, disclose bubble memories including a single level of electrically-conducting material. Bipolar pulses are applied to the electrically-conducting material which responds to provide, in effect, two phases of the three-phase operation necessary for unidirectional movement of bubbles. In these last two instances, the bubble layer itself is formed into a discrete strip patterned to provide offset rest positions for bubbles and thus operating as a "third-phase" of propagation.
The field-access type memory, on the other hand, employs only a pattern of permalloy elements, or equivalent such as ion-implanted regions, which defines bubble propagation paths in an adjacent bubble layer. The elements are arranged in a plane closely spaced apart from the bubble layer and are responsive to a substantially uniform magnetic field reorienting cyclically in the plane of bubble movement to provide localized moving magnetic pole patterns. The bubbles follow the pole pattern as is well understood. Half-disc, half-chevron, T-bar, and chevron-shaped elements are all familiar geometries characteristic of field-access type memories.
The problem with the field-access arrangement is that relatively costly field coils are necessary to provide a rotating in-plane field to move bubbles and that that field has to be fairly uniform over a relatively large area in which bubble movement occurs. Further, high speed operation is increasingly difficult to achieve because of the difficulty of supplying the requisite volt-ampere product to the coils at increasingly higher frequencies.
On the other hand, conductor-access arrangements require intricate patterns of conductors which, for the only working systems, comprise multilevel circuitry which have relatively high power requirements. Such conductor patterns are too susceptible to short circuits, open circuits, and nonuniformities to allow the realization of circuits of commercially-acceptable size. But conductor-access circuits potentially are capable of relatively high operating speeds because they need not provide uniform fields over large areas. Thus, conductor-access circuits appear attractive. The specific problem then is to realize a conductor-access arrangement for bubble memories which does not succumb to high power requirements or to current nonuniformity problems.